The present invention relates to a novel aluminium hydrate, to a process for preparing said hydrate and to its use for the preparation of catalysts or catalyst supports.
Aluminium hydrates are used to prepare catalysts or catalyst supports. The preparation normally consists of forming the aluminium hydrate, then calcining it to transform it to alumina. The properties of the aluminium hydrate influence the characteristics of the alumina obtained and, as a result, the properties of the catalysts and catalyst supports. High dispersibility of the aluminiun hydrates in water is a desirable property as it enables catalysts or catalyst supports with advantageous properties to be prepared:
firstly, a dispersible aluminium hydrate leads to catalyst supports containing no alumina agglomerates or aggregates. Thus homogeneous supports are obtained;
then, with a dispersible aluminium hydrate, the forming conditions are milder (for example the amount of acid is lower) so a large total pore volume can be retained;
finally, catalysts or catalyst supports from a dispersible aluminium hydrate have a unimodal pore distribution with not meso and/or macropores (the terms xe2x80x9cmesoporexe2x80x9d and xe2x80x9cmacroporexe2x80x9d means any pores with a size over the median diameter). In certain applications, such as hydrotreatment in oil refineries, the absence of these meso and/or macropores is desired: if the pore diameter is low (100 xc3x85), the metals do not enter the support, preventing deactivation. Further, if the pore distribution is narrow, only certain molecules enter into the support which means that molecular sieving can be carried out. Further, the coking effects are limited and the absence of macropores increases the mechanical strength of the support.
Thus one aim of the present invention is to provide an aluminium hydrate with high dispersibility in water.
A further aim is to provide a process for preparing said aluminium hydrate.
Thus in a first aspect, the invention concerns an aluminium hydrate with a water dispersibility of at least 80% by weight for a diameter of 0.3 xcexcm or less.
The invention also concerns processes for preparing such an aluminium hydrate. In one implementation, the aluminium hydrate is prepared without back-mixing, using the following steps:
1. mixing a source of an aluminium acid and a base or a basic source of aluminium and an acid, to precipitate an aluminium monohydrate;
2. maturing;
3. filtering;
4. washing; and
5. drying.
In a further implementation, the aluminium hydrate is prepared with back-mixing, using the following steps:
1xe2x80x2. mixing aluminium sulphate and a base at a pH in the range 6 to 8.5 in a stirred reactor with back-mixing, at a temperature in the range 50xc2x0 C. to 95xc2x0 C.;
2xe2x80x2. maturing at a pH in the range 9 to 11 and at a temperature in the range 60xc2x0 C. to 90xc2x0 C.;
3xe2x80x2. filtering;
4xe2x80x2. washing; and
5xe2x80x2. drying.
Finally, the invention concerns the use of this aluminium hydrate for preparing catalysts or catalyst supports.